1. Field of the Invention
The present invention relates generally to corrosion testing of steam generator tubes and, more particularly, to a rotary union for rotatably supporting an ultrasonic probe operable to nondestructively determine the amount of corrosion experienced by the tubes.
2. Description of the Prior Art
Heat produced by fission in a nuclear reactor core of a nuclear power plant is transferred to a primary reactor coolant flowing through the reactor core. The primary reactor coolant then flows through steam generators of the nuclear power plant where it transfers the heat to a secondary feedwater which is transformed thereby into steam. The steam is used to generate electricity by driving a conventional steam turbine-electrical generator apparatus.
Each steam generator has a large bundle of tubes. The high temperature primary reactor coolant flows through the interior of the tubes in heat exchange relationship with the feedwater flowing along the exterior of the tubes. The primary reactor coolant flowing through the steam generator tubes is a source of corrosion of the walls of the tubes which reduces wall thickness and can eventually lead to wall perforations.
As a result, long term corrosion tests of steam generator tubes are conducted in order to understand the corrosion generating mechanism and to determine typical steam generator tube corrosion rates.
One type of test performed to determine the amount of corrosion experienced by a steam generator tube is a nondestructive test which utilizes an ultrasonic probe placed inside the tube. The tube is filled with water to allow an ultrasonic wave generated by the probe to travel from the probe through the water and into the tube wall. The reflected ultrasonic wave is monitored by the same probe. The time required for the ultrasonic wave to complete a round trip in the tube wall is proportional to the tube wall thickness. The relationship between wave travel time and tube thickness is: EQU T=(vt)/2
where: PA0 T=tube wall thickness (inches) PA0 v=velocity of ultrasound in metal PA0 t=travel time (micro-seconds).
If it is desired to nondestructively determine the amount of corrosion which has occurred over a selected section of a steam generator tube, the ultrasonic probe must be passed through the inside of the tube over the section being tested. In addition, in order to determine the amount of corrosion which has occurred around a selected circumference of the tube, the ultrasonic probe must be rotated in a complete circle. Since the tube is positioned vertically within the steam generator shell, the ultrasonic probe must be introduced into the bottom of the tube which terminates at the open lower end portion of the steam generator shell and thereafter extended upwardly into the interior of the tube and rotated in sweeping fashion to nondestructively determine the amount of corrosion which has occurred over the entire section of tube.
In practice, the ultrasonic probe utilized to nondestructively determine the amount of corrosion experienced by a steam generator tube has heretofore been supported by an adaptor which has a hollow interior through which passes both an electrical cable for providing an electrical signal to the ultrasonic probe and water to be utilized by the probe. The adaptor itself is maintained in position within the open lower end portion of the steam generator adjacent to the bottom of the tube to be tested. A connector element in the form of a hollow pipe is used to secure the ultrasonic probe to the adaptor and also acts as a conduit for the electrical cable and water supplied to the probe. In order to extend the ultrasonic probe upwardly into the interior of the tube under test, additional sections of pipe are interposed between the ultrasonic probe and the adaptor.
Although this adaptor has been used in the past to support the ultrasonic probe, its use has caused problems for operating personnel. Since the adaptor itself is a rigid assembly, the entire adaptor must be rotated in order to allow the ultrasonic probe to sweep the entire inside circumference of the tube. It has been found that this rigid adaptor, due to its construction, interferes with the steam generator open end portion outer shell, particularly during ultrasonic inspection of the outer ring of steam generator tubes. In addition, if it is desired to add additional sections of pipe between the ultrasonic probe and the adaptor to extend the probe upwardly into the tube, the water supply to the probe must be disconnected and the electrical cable disconnected from its electrical signal supply and pulled out of the adaptor. Only after the additional sections of pipe are added can the water supply to the probe be reconnected and the electrical cable be reinserted through the pipes and adaptor to be reconnected to the electrical signal supply. It is apparent that this requirement of having to continually disconnect and then reconnect the electrical signal and water supplies from the ultrasonic probe in order to add sections of pipe between the probe and adaptor to extend the probe upwardly into the interior of the tube under test greatly increases overall tube inspection time and the amount of work required to be performed by test personnel.
Consequently, a need exists for an improved adaptor in the form of a rotary union for supporting an ultrasonic probe which is capable of accepting sections of pipe to extend the probe upwardly into the tube under test without requiring the water and electrical signal supplies to the probe to be disconnected. Utilizing a rotary union to support the ultrasonic probe will also provide the added benefit of permitting the ultrasonic probe to be rotated while the body of the union remains stationary in order to eliminate the steam generator interference problems experienced by the adaptor presently used.